Can you point to where they have said so? The only one that comes to mind is Thiel's quote from 2009 about democracy being incompatible worth freedom (the populace will vote to remove freedoms, e.g. try to ban AI or other technological advances and whatnot). But pointing out flaws in democracy is a far cry from actual wanting to get rid of democracy.

If he's stated an actual intent to end democracy in the US, it'd be good to cite that.